The removal of toxic vapors from flowing air streams has relied upon adsorption, absorption, photoionization, catalysis, or incineration. To date, charcoal adsorption has been the proven technology for ambient temperature purification of contaminated air streams, while incineration has been the proven technology for high temperature purification of contaminated air streams. However, charcoal filters suffer from a short lifetime, limited adsorptive capacity for toxic compounds, desorption of toxic compounds from the filter, and selectivity for only certain classes of compounds. Additionally, a charcoal filter must be used in conjunction with a particulate filter to collect toxic aerosols. Particulate filters also have a limited lifetime resulting from clogging which restricts air flow. Catalytic technology applicable to broad chemicl processing such as air purification requires high temperatures to achieve efficient decomposition and is subject to poisoning. Many devices such as incinerators, infrated reactors, catalytic reactors, and thermal equilibrium plasma devices especially direct current arcs, rely on thermal energy to induce chemical reactions. All of the exposed surfaces and the air stream must be raised to a high equilibruim temperature by the transfer of thermal energy. Chemical reactions are promoted when sufficient thermal energy forces the atoms and molecules to `collide`.
J. M. Henis (U.S. Pat. No. 3,983,021) discloses a packed electrical discharge device for decomposing nitrogenous oxides (NO.sub.x) in a gas stream composed predominantly of nitrogen. The chemical conversion efficiency of the process is low. This device would not be useful as a plasma air purification system because as the oxygen concentration is increased to levels found in breathable air, the plasma-formed by-products such as O.sub.3 increase to toxic levels in the reactor effluent. R. McNabney (U.S. Pat. No. 3,734,846) utilizes a fluidized bed silent electrical discharge for the conversion of oxygen to ozone. Similarly, Heinemann (U.S. Pat. No. 4,774,062) improves the power efficiency of a conventional ozonizer by reducing the thickness of the dielectric barriers on the electrodes. It should be mentioned that the gas phase decomposition does not occur rapidly with ozone as the oxidant. Therefore, any ozone generating corona discharge will not efficiently decompose toxic material. These devices suffer low chemical conversion efficientcies because of their low activity plasmas. Their requirement for dielectric barriers at the electrodes of these devices severely limits the power density of the plasmas generated. In addition, these low activity plasmas generate high concentrations of toxic by-products (especially ozone) when used to process air.
Several devices will resistively heat a catalytic mass (J. Koetschet, U.S. Pat. No. 1,400,959), or dielectrically heat with high frequency (Van Der Lande, U.S. Pat. No. 2,163,898) but these systems eventually poison and lose their functionality. Some authors have used photoactivation of catalysts (Mitscherling, U.S. Pat. No. 2,003,303) to accomplish chemical processing. Catalytic technology applicable to broad chemical processing applications requires high temperatures to achieve efficient decomposition. It should be noted that the usable lives of systems using conventional adsorbent and catalyst technologies are severly limited by saturation or poisoning. An important distinction between adsorbent-based or catalyst-based air purification systems and plasma systems is that air processed in adsorbent or catalyst systems does not yield the characteristic plasma air by-products such as nitrogenous oxides and ozone.